Digital signal transmission and reception

ABSTRACT

A digital signal transmitter in which multiple data streams are each transmitted by modulation of a respective frequency band within one of a group of frequency channels, the frequency bands each occupying no more than a predetermined maximum bandwidth less than or equal to the channel width; comprises means for transmitting at respective frequency positions within each frequency channel, one or more instances of band information defining the frequency bands corresponding to all of the data streams carried within that frequency channel, the one or more instances being arranged so that any portion of the frequency channel equal in extent to the predetermined maximum bandwidth includes at least one instance of the band information.

FIELD OF THE INVENTION

This invention relates to digital signal transmission and reception.

DESCRIPTION OF THE PRIOR ART

Digital signals are transmitted in applications such as digitaltelevision broadcasting. Standards such as the so-called DVB standardshave existed since the 1990s, and provide a range of quadratureamplitude modulation (QAM) schemes for broadcast services, along withformats for the transmission of accompanying control data and metadata.These standards define both the radio frequency (RF) techniques used tocarry the data and the way in which the data representing differentbroadcast services is organised into packets and streams fortransmission.

The DVB standards are described extensively elsewhere, so only a briefsummary will now be given, with reference to the standards relating tothe transmission of broadcast cable services, although it will of coursebe appreciated that similar considerations can apply to (for example)digital satellite services and terrestrially broadcast services.

In basic terms, the video data, audio data and accompanying datacorresponding to a broadcast programme are multiplexed into an MPEG-2Programme Stream (PS). One or more PSs are multiplexed to form atransport stream (TS) formed as a sequence of fixed length data packets.The bit rate of the TS can range between about 6 Mbit/s and 64 Mbit/sdepending on parameters such as the modulation scheme in use (16QAM to256QAM for example) and the bandwidth of the broadcast channel whichwill be used to carry the TS.

With current technology, one broadcast channel (with a bandwidth of afew MHz-up to 8 MHz) carries one TS. The TS includes packetisedprogramme data (video, audio etc.) and packetised control data definingthe different programmes carried by that TS. Optionally, a so-callednetwork information table (NIT) is also carried, which providesinformation about the physical network, such as channel frequencies,service originator and service name.

There is a growing demand not only for more digital television servicesbut also for higher quality (in terms of picture and audio resolution)services. This demand imposes pressure on the digital payload carried byeach channel. It is a constant aim to use the available broadcastspectrum efficiently and flexibly.

SUMMARY OF THE INVENTION

This invention provides a digital signal transmitter in which multipledata streams are each transmitted by modulation of a respectivefrequency band within one of a group of one or more frequency channels,the frequency bands each occupying no more than a predetermined maximumbandwidth less than or equal to the channel width;

in which the transmitter comprises means for transmitting at respectivefrequency positions within each frequency channel, one or more instancesof band information defining the frequency bands corresponding to all ofthe data streams carried within that frequency channel, the one or moreinstances being arranged so that any portion of the frequency channelequal in extent to the predetermined maximum bandwidth includes at leastone instance of the band information.

Embodiments of the present invention conveniently allow channels ofwidth greater than the receiver bandwidth, or the width of anyindividual payload stream, to be used, by providing a mechanism forindicating to a receiver where, within a frequency channel, it shouldalign its receiver bandwidth in order to receive a desired data stream.

Further respective aspects and features of the present invention aredefined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings, in which:

FIG. 1 schematically illustrates a digital signal transmission system;

FIG. 2 schematically illustrates a previously proposed data transmissionframe;

FIG. 3 schematically illustrates a data transmission frame according toan embodiment of the present invention;

FIG. 4 schematically illustrates an L1 data packet;

FIG. 5 schematically illustrates a network information table;

FIG. 6 schematically illustrates a data superframe;

FIG. 7 schematically illustrates a receiver apparatus;

FIG. 8 schematically illustrates a transmission method; and

FIG. 9 schematically illustrates a reception method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a digital signal transmission system comprisesa transmitter 10 and a receiver 20, linked by a transmission link 30. Inthis example, the transmission link is a wired link (which termencompasses electrically conductive wires and optical fibres), and thesystem of FIG. 1 is arranged to provide a cable television service. Ingeneral terms, except where differences are described, the apparatusoperates in accordance with the DVB-C standards.

An optional return channel 40, by which data such as purchasing data orviewing statistics can be carried from the receiver to the transmitter,is also provided. The return channel is conventional and will not bedescribed further here.

The present techniques are not limited to cable systems. Correspondingtechniques may be used in other transmission systems such as terrestrialor satellite systems.

The transmitter comprises a number of programme multiplexers 100, onefor each programme data stream to be transmitted. These multiplex video,audio and data streams relating to a programme into an MPEG-2 programmestream (PS). The PS is multiplexed by a transport multiplexer 110 withPSs for other programmes to form an MPEG-2 transport stream (TS). A TSis a basic digital stream carried by the system, and can have a bit rategenerally in the range of about 6 to about 64 Mbit/s.

The TS is multiplexed with other TSs (or other input streams such asso-called Generic Encapsulated Streams, Generic Continuous Streams orGeneric Fixed-Length Packetised Streams) and also network information(to be described later) by a multiplexer 120, with the resulting databeing passed to an encoder and modulator 130.

The encoder and modulator 130 encompasses such functions as packetising,channel coding, data interleaving (by time and/or frequency), wordlength conversion, differential coding, QAM coding, frame generation,base-band filtering and radio frequency (RF) modulation such as OFDMmodulation in which each payload stream is carried by modulated groupsof (generally) adjacent sub-carriers. Except where described, thesefunctions correspond to known functions of a DVB-C transmitter. (In apractical system there may well be multiple transmitters coupled to acombiner to generate a combined modulated RF signal). The modulated RFsignal is passed to the cable 30 for transmission to one or more (andgenerally a large number of) receivers 20.

The transmitter 10 operates under the control of a controller 140. Thefunctions carried out by the controller 140 (e.g. preparation of NIT andband information data) will be described below.

The receiver 20 will be described in more detail below, particularlywith reference to FIG. 7. For now, it is sufficient to note that thereceiver operates in accordance with control information to demodulateand decode a required data stream—for example, a particular TS—from thetransmitted signal.

In operation, the transmitter 10 operates at a number of frequencychannels. These lie generally within the range of about 40 to about 800MHz. But the present techniques could apply to an arrangement havingonly one frequency channel. Within each channel, data are transmitted byOFDM modulation of multiple sub-carriers.

In previous systems the channels had a fixed width, for example 8 MHz,with each channel being adjacent in frequency (perhaps with a smallguard band) to the next channel. FIG. 2 schematically illustrates a datatransmission frame 200 within such a channel in a previous DVB-C system.

In FIG. 2, time is represented in a downwards vertical direction andfrequency in a horizontal direction (both with respect to theorientation of the drawing). In a time order, the data frame 200comprises:

(a) preamble data 210, which amongst other possible functions acts tomark the start of the data frame;(b) layer 1 (L1) data which, amongst other possible functions identifiesthe subsequent data payload and provides physical layer parameters foruse by receivers relating to the encoding of the data payload; and(c) payload data—in this case a TS.

The data frame 200 occupies the whole channel width (8 MHz in thisexample). The receiver bandwidth would also correspond to this channelwidth. Accordingly, the receiver can be successfully aligned with achannel of this type simply by specifying the centre frequency of thechannel. This centre frequency information can be provided in anoptional Network Information Table (NIT) broadcast as part of each TS.Alternatively, the centre frequencies can be detected by a receivercarrying out an automated channel “sweep”, in which the entire availablefrequency range is scanned to detect broadcast channels.

The present embodiment allows channels of different widths (e.g. 8, 16or 32 MHz) to be used. However, the receiver bandwidth (and,correspondingly, the maximum allowed bandwidth by which a single payloaddata stream such as a TS can be carried) remains the same as with theprevious systems, e.g. 8 MHz. In other words, the predetermined receiverbandwidth is less than or equal to the channel width. The techniquesalso allow multiple TSs or other types of payload to be carried within asingle channel. FIG. 3, illustrating a DVB-C2 data transmission frameaccording to an embodiment of the present invention, will be used toexplain how this technique operates.

Once again, time is represented in a downward vertical direction andfrequency in a horizontal direction. A data frame 300 is (in thisexample) 32 MHz wide and starts with preamble data 310 similar to thepreamble data 210.

After the preamble data there follow multiple instances of L1 data, atdifferent frequency positions within the channel. The particularcontents of the L1 data will be explained below, but with reference tothe example of FIG. 3 it should be noted that 8 such instances areprovided within a 32 MHz channel. In other words, each instance of theL1 data is provided by a group of adjacent OFDM sub-carriers which,taken as a group, occupy a band 4 MHz wide, though more generally eachinstance could be less than or equal to half of the receiver bandwidth,and there could be a correspondingly higher number of instances if thebandwidth of each instance was lower. (In fact, to avoid problems ofattenuation at the extreme edges of filter passbands defining thechannels, each instance of L1 would be very slightly less than 4 MHzwide, but for the purposes of the present description they will bereferred to as being 4 MHz wide).

The eight instances of the L1 data (in this example) within a single 32MHz channel are identical and, for convenience, are transmitted at thesame time. The reason that the L1 data is transmitted in a bandwidth nogreater than half that of the receiver bandwidth is that wherever the 8MHz receiver bandwidth is aligned within the 32 MHz channel, thereceiver bandwidth is bound to encompass at least one complete instanceof the L 1 data.

The multiple instances of the L1 data need not (when consideredtogether) fill the whole channel width. There could be frequency gaps orguard bands between them. The constraint that the width of an individualinstance should be less than or equal to half of the receiver bandwidthassumes that the receiver bandwidth is less than the channel width; ifthe receiver can in fact encompass the whole channel, then in principleonly one instance would be required, and that constraint would notapply.

As before, there are two routes to the receiver locating a channel. Oneis via the NIT, and the other is through a frequency sweep as describedabove.

The NIT in this embodiment defines the centre frequency for each channelrather than defining frequency ranges for individual TSs within thatchannel. Each TS carried by a channel is described by the centrefrequency of the channel, rather than by the centre frequency of thefrequency band carrying the data stream representing that TS. In orderto find the centre frequency of the frequency band for the relevant TS,the receiver first aligns its receiver bandwidth with the centrefrequency 340 of the channel, then detects the next available instanceof the L1 data (which in this case would be the next availabletransmission of either the instance 321 or the instance 322), thendetects from the received L1 data the centre frequency and otherreceiver parameters (e.g. QAM parameters, identity of the sub-carrierstreams, bandwidth etc) of the required TS. For example, if the requiredTS for a particular PS is the TS 350, the L1 data for that PS wouldspecify at least (a) the TS; (b) the TS centre frequency 360; and (c)receiver parameters for the TS. Knowing the centre frequency andbandwidth, the receiver would align its receiver bandwidth 370 to ensurethat it encompasses the band occupied by that TS.

If the required channel is located by a frequency sweep, then themechanism for locating a TS is similar, in that the receiver aligns itsbandwidth with any position within the channel and detects an instanceof the L1 data. From this, the receiver can extract all of theinformation needed to receive the required TS, in the same way as justdescribed.

The payload data 330 follows the L1 data. Multiple TSs can be carried bya single channel, along with other types of data such as IP data 332—anexample of the more general data type known as “generic streamencapsulation” or GSE.

FIG. 4 schematically illustrates an instance of the L1 data. The L1 datahas various existing functions within the DVB-C and MPEG standards, buthere the specific functions to be described are that for each TS (e.g.the TSs: TS1 . . . 4), the L1 data defines: the centre frequency of eachTS; the bandwidth of the TS; and receiver parameters for that TS.

FIG. 5 schematically illustrates a network information table (NIT). TheNIT is transmitted as a data stream with a unique programme identifierPID. If it is transmitted at least once in each TS, it can therefore beextracted by reference to that PID. It is considered optional andproprietary in the context of the DVB-C standards, and as such cancontain various types of data. But amongst such other possiblefunctions, in the present context the NIT serves to identify the channelcentre frequency (and, optionally, other parameter data) for each TS.

FIG. 6 schematically illustrates a data superframe relating to onechannel.

The superframe is formed of multiple frames (such as the frame 300 shownin FIG. 3), preceded by preamble data and followed by postamble data. Afurther superframe would follow directly after the postamble data of acompleted superframe. Each frame contains the multiple instances of theL1 data (i.e. spread across the frequency range relating to that channeland repeated in time at least once during the frame), which means thatthe instances of the L1 data are repeated periodically in time duringthe superframe. To put the periods into context, the frames have lengthsmeasured in milliseconds, whereas a superframe can have a lengthmeasured in hours or even longer. So, the delay involved in establishingthe correct TS and receiver parameters for a particular PS is of theorder of one frame length.

The system is constrained so that changes to the L1 data occur only atsuperframe boundaries. It is possible for new values of the L1 data tobe transmitted in the last few frames of a superframe, but not to haveeffect until the superframe boundary, in order to allow the receiver toprepare for (say) a retuning operation.

FIG. 7 schematically illustrates a receiver apparatus. The incomingcable signal is supplied to a data receiver which comprises a tuner(having in this example an 8 MHz bandwidth, though this may beadjustable as described below), a QAM demodulator and a channel decoder,which carries out such known operations as differential decoding, wordlength conversion, de-interleaving and the like to generate output data.

The data signal output by the data receiver 400 is passed to a decoder410 and a parameter detector 420 associated with a parameter store 430.

The parameter detector 420 carries out the functions of detectingchannel details from the NIT or from the sweep and detecting TS detailsfrom the L1 data. All of these details are stored in the parameter store430 and used to control the data receiver 400. The way in which this iscarried out will be summarised with respect to FIG. 9 below. A dashedline connection between the parameter detector 420 and the data receiver400 indicates the possibility mentioned above of the parameter detectorpassing advance details towards the end of a superframe of changes inreceiver parameters due to take effect at the end of the superframe.

The decoder 400 operates to decode the required PS stream once theappropriate receiver parameters have been set.

The data receiver 400 may have a variable bandwidth, within certainlimits. For example, the data receiver 400 may have a bandwidth that isselectable between 8 MHz and 7 MHz—possibly to allow for legacycompatibility with different instances of previous DVB-C systems. Such afeature can in fact be used in connection with the present techniques,so that once the L1 data has defined parameters to receive the requireddata stream, the data receiver can set its receiver bandwidth to thelowest (or simply a low) setting (from amongst those values available tothe data receiver) which still encompasses the required data stream,allowing of course for so-called roll-off which is a lessening of thedata receiver's response at the edges of the data receiver's bandwidth.Where such a feature is used, the data receiver can for example set itsbandwidth back to a higher level (if that is indeed necessary given thewidth of each instance of the L1 data) whenever the L1 data specificallyneeds to be accessed.

FIG. 8 is a schematic flow chart showing the operation of a transmitter.

In FIG. 8, steps 500 and 510 are (in this example) carried out by thecontroller 140, in which channel information (corresponding to the NITdata described above) and instances of band information (correspondingto the instances of the L1 data described above) are generated. Notethat the generation of the NIT data is optional; the channels can beidentified by a frequency sweep instead. At a step 520 these data aretransmitted (a step carried out by the encoder and modulator 130).

FIG. 9 is a schematic flow chart illustrating the operation of areceiver.

In FIG. 9, at a step 600 a required channel (for a desired output datastream) is detected. This could be by the parameter detector 420examining the NIT data from within a currently detected channel, or itcould be by a frequency sweep and then examination of the L1 data in thedetected channels by the parameter detector.

At a step 610, the parameter detector 420 stores appropriate parametersfor the required channel in the parameter store 430, and the datareceiver 400 aligns its receiver bandwidth to the requiredchannel—preferably to the centre frequency of the required channel. At astep 620 the parameter detector 420 receives band information(corresponding to the L1 data) for the required output data stream, viathe data receiver, and stores appropriate parameters in the parameterstore 430. The data receiver aligns its receiver bandwidth to the centrefrequency of the required band (i.e. the portion of the channelcontaining the required payload data) at a step 630.

The payload data can then be received and decoded by the decoder 410 toproduce the required output stream.

Note that in fact the L1 data may define a receiver centre frequencywhich is not in fact the centre frequency of the band containing therequired payload data, but which is still such that the receiver, whencentred around that receiver centre frequency, will still receive all ofthe sub-carriers relating to the required payload data. One example ofwhy this might be done is where the required band is close to an edge ofthe channel, and there is a potentially interfering signal outside thechannel but adjacent or near to that edge of the channel. Here, it couldbe beneficial for the receiver centre frequency defined by the L1 datato be offset away from the edge of the channel, so that the receiver isless likely to receive the interfering signal.

In the meantime the parameter detector continues to monitor the bandinformation contained in successively transmitted instances of the L1data at a step 640. In an embodiment, the bandwidth of the L1 data issuch that whatever the centre frequency of the data receiver, it willalways receive at least one full instance of the L1 data. However, inanother embodiment, if the bandwidth of the L1 data and the receiverbandwidth and centre frequency are such that only (say) the highersub-carriers of one instance of the L1 data and the lower sub-carriersof the next instance of L1 data are received, the data receiver can bearranged so as to re-order those received sub-carriers (e.g. after anFFT stage on the receiver side) into the order corresponding to acomplete instance of the L1 data for demodulation and decoding.

At a step 650, if any changes to the band information are detected,control passes back to the step 630 so that the relevant receiverparameters can be adjusted. Otherwise, control passes back to the step640 so that the monitoring process can continue.

The parameter store can be used to improve the operation of the receiverin a further way. Historically, a broadcast channel has carried one TS.The centre frequency of this channel was included in the NIT (ifpresent) or was discovered by the receiver as part of a frequency sweep,and was stored in a database within the receiver against that TS. When areceiver needed to receive a TS, it would consult its database to findthe centre frequency for that TS and send that information to the tuneras a tuner parameter.

In the embodiments described here, a TS can occupy a frequency bandforming a subset of a frequency channel. If the receiver followed thehistorical approach described above, it would store the centre frequencyof the channel and align its receiver bandwidth to that centre frequencywhenever it required to receive a particular TS which makes up part (orall) of that channel. It would then need to read the L1 data asdescribed above to find out the exact frequency band used by the TS,perhaps necessitating retuning to that frequency band (within the samechannel) to be able to receive the correct frequency band. This two stepoperation would be required every time it needed to receive a TS.

Embodiments of the present invention provide a different way ofperforming this tuning operation. The first time that a TS is required,the receiver is tuned to the centre frequency of the channel so as todiscover the frequency parameters for that TS, before (if necessary)retuning to the correct frequency band. However, once the frequency bandfor a TS (and, potentially, other receiver parameters) has beendiscovered from the L1 data, it is stored in the parameter store as alook-up record relating to that TS. This means that the next time thatthe receiver needs to receive that TS, it uses the information alreadystored in the parameter store to align directly with the frequency bandused by that TS. This is represented by a step 605 in FIG. 9. Of course,the frequency band may have been modified since the details were storedin the parameter store, so the checks relating to the steps 640 and 650are still carried out to verify that the receiver parameters detailed inthe L1 data have not changed.

Note that because the instances of the L1 data within a channel areidentical, the steps 640 and 650 can be carried out to verify parameterscached in the parameter store whenever the receiver is tuned to any TSin that frequency channel; not just when the receiver is tuned to aparticular TS.

In this way, the receiver is caching (in the parameter store) the lastknown parameters for a TS, rather than the channel as notified in theNIT.

It will be appreciated that the apparatus and methods described abovemay be implemented, at least in part, by computing or data processingapparatus operating under the control of appropriate software commands.Such software, and data carriers which serve to provide such software,are therefore considered as embodiments of the invention.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

1. A digital signal transmitter in which multiple data streams are eachtransmitted by modulation of a respective frequency band within one of agroup of one or more frequency channels, the frequency bands eachoccupying no more than a predetermined maximum bandwidth less than orequal to the channel width; in which the transmitter is operable totransmit at respective frequency positions within each frequencychannel, one or more instances of band information defining thefrequency bands corresponding to all of the data streams carried withinthat frequency channel, the one or more instances being arranged so thatany portion of the frequency channel equal in extent to thepredetermined maximum bandwidth includes at least one instance of theband information.
 2. A transmitter according to claim 1, being operableto transmit for each data stream, channel information defining therespective frequency channel.
 3. A transmitter according to claim 2, inwhich at least one of the frequency channels carries channel informationrelating to data streams carried by one or more other frequencychannels.
 4. A transmitter according to claim 2, in which the channelinformation defines a centre frequency of the respective frequencychannel.
 5. A transmitter according to claim 1, arranged to transmit theband information repetitively.
 6. A transmitter according to claim 5,arranged to transmit each instance of the band information at periodictime intervals.
 7. A transmitter according to claim 6, in which the datacarried by a channel are arranged as data frames, each instance of theband information being transmitted at least once in each data frame. 8.A transmitter according to claim 6, arranged so that all instances ofthe band information are transmitted at substantially the same time. 9.A transmitter according to claim 7, in which: multiple consecutive dataframes are arranged as a data superframe; and the frequency bands usedby the data streams are constrained so that changes in the bandinformation may not occur within a superframe.
 10. A transmitteraccording to claim 1, in which each instance of the band informationoccupies a bandwidth no greater than half of the predetermined maximumbandwidth.
 11. A transmitter according to claim 1, in which thepredetermined maximum bandwidth is 8 MHz and the channel width isselected from the group consisting of 8, 16 and 32 MHz.
 12. Atransmitter according to claim 1, in which the predetermined maximumbandwidth is less than the width of a frequency channel.
 13. Atransmitter according to claim 1, in which at least some of the datastreams represent video and/or audio signals.
 14. A digital signaltransmission system comprising: a transmitter according to claim 1; anda digital signal receiver for receiving a required data stream from adigital signal transmitted by the transmitter, the receiver comprising adata receiver having a receiver bandwidth equal to or greater than thepredetermined maximum bandwidth; in which the data receiver is arrangedto align its receiver bandwidth with a channel transmitted by thetransmitter so as to receive, from within that channel, an instance ofthe band information, and then in response to the received bandinformation, to align its receiver bandwidth so as to encompass thefrequency band of the desired data stream.
 15. A system according toclaim 14, comprising: a parameter store arranged to retain theinformation to align the receiver bandwidth so as to encompass thefrequency band of the required data stream; and in which if the datastream is required again then the data receiver is arranged to align itsreceiver bandwidth according to the retained information of the lastknown frequency band of the required data stream.
 16. A system accordingto claim 14, comprising an electrical and/or optical cable connection tocarry the transmitted digital signal from the transmitter to thereceiver.
 17. A digital signal transmission method in which multipledata streams are each transmitted by modulation of a respectivefrequency band within one of a group of one or more frequency channels,the frequency bands each occupying no more than a predetermined maximumbandwidth less than or equal to the channel width; the method comprisingthe step of: transmitting at respective frequency positions within eachfrequency channel, one or more instances of band information definingthe frequency bands corresponding to all of the data streams carriedwithin that frequency channel, the one or more instances being arrangedso that any portion of the frequency channel equal in extent to thepredetermined maximum bandwidth includes at least one instance of theband information.
 18. A data carrier on which is stored computersoftware having program code which, when executed by a computer, causesthe computer to carry out a method according to claim
 17. 19. A digitalsignal receiver for receiving a required data stream from a digitalsignal carrying multiple data streams by modulation of respectivefrequency bands each within a respective one of a group of frequencychannels, the frequency bands occupying no more than a predeterminedreceiver bandwidth less than or equal to the channel width; the digitalsignal carrying at respective frequency positions within each frequencychannel and repeated from time to time, multiple instances of bandinformation defining receiver parameters including at least thefrequency bands corresponding to all of the data streams carried withinthat frequency channel, the instances being arranged so that any portionof the frequency channel equal in extent to the predetermined receiverbandwidth should include at least one instance of the band information;in which the data receiver is operable to align its receiver bandwidthwith a channel so as to receive, from within that channel, an instanceof the band information, and then in response to the received bandinformation, to align its receiver bandwidth so as to encompass thefrequency band of the required data stream.
 20. A receiver according toclaim 19, in which the data receiver comprises: a detector to detect anychanges in receiver parameters indicated by a subsequent instance of theband information; and a receiver parameter adjuster to alter itsreceiver parameters for that data stream in response to such changes.21. A receiver according to claim 19, comprising: a store for storingreceiver parameters to control operation of the receiver; a storeupdater to update the store in response to receiver parameters indicatedby received instances of the band information.
 22. A receiver accordingto claim 21, in which, if receiver parameters relating to a requireddata stream are present in the store, the receiver is arranged to alignits receiver bandwidth according to the receiver parameters stored inthe store.
 23. A receiver according to claim 19, comprising a detectorto detect, from within a currently detected channel, channel informationdefining the respective frequency channel for each data stream.
 24. Adigital signal reception method for receiving a required data streamfrom a digital signal carrying multiple data streams by modulation ofrespective frequency bands each within a respective one of a group offrequency channels, the frequency bands occupying no more than apredetermined receiver bandwidth less than or equal to the channelwidth; the digital signal carrying at respective frequency positionswithin each frequency channel and repeated from time to time, multipleinstances of band information defining receiver parameters including atleast the frequency bands corresponding to all of the data streamscarried within that frequency channel, the instances being arranged sothat any portion of the frequency channel equal in extent to thepredetermined receiver bandwidth should include at least one instance ofthe band information; the method comprising the steps of: aligning thereceiver bandwidth with a channel so as to receive, from within thatchannel, an instance of the band information; and in response to thereceived band information, aligning the receiver bandwidth so as toencompass the frequency band of the required data stream.
 25. A datacarrier on which is stored computer software having program code which,when executed by a computer, causes the computer to carry out a methodaccording to claim 24.